Absorption heating and cooling systems, particularly air conditioners and heat pumps in which ammonia is the refrigerant and water the absorbent, are well-known as efficient and cost-effective alternatives to conventional vapor compression air conditioning and heat pump systems, as well as conventional furnaces. Cost savings of the aqua-ammonia systems driven by combustion of natural gas are significant compared to the conventional systems driven by more expensive electrical power. High-efficiency generator absorber heat exchange cycle (GAX) apparatus such as disclosed in Modahl, et al. xe2x80x9cEvaluation of a Commercial Advanced Absorption Heat Pump Breadboardxe2x80x9d, 1988, and in U.S. Pat. Nos. Re. 36,684 and 5,367,884 are examples of further improvements in aqua-ammonia absorption systems. However, such ammonia refrigerant systems require the use of a hydronic heat transfer coupling or other heat exchange assembly for delivering heating and cooling to the living space because ammonia is excluded from use indoors in direct-expansion evaporators or indoor condensers. Ammonia is classified as a safety Group B2 refrigerant by ASHRAE and a Group II gas by UL. Moreover, local and national codes and regulations prohibit the use of ammonia in equipment exposed to indoor or other enclosed areas intended for occupation, except for small quantities.
In addition to aqua ammonia absorption systems, other heating or cooling systems using a working fluid that is restricted or prohibited for use in the conditioned space must have an isolated heat transfer assembly for transferring heating and cooling to the load. Heat exchange assemblies for coupling aqua-ammonia systems, and other refrigeration or heating systems, include pumped sensible heat loops, and phase-change loops. Pumped sensible heat loops utilize heat transfer fluids such a water, water-glycol solutions, brines, or oils. Hydronic (water based) loops are typically used with unitary aqua-ammonia absorption systems. Both phase-change and pumped systems have disadvantages. Hydronic heat transfer couplings commonly used for transferring cooling and heating from an aqua-ammonia system into a conditioned space utilize a pumped loop containing a solution of water and a heat transfer fluid such as ethylene glycol or propylene glycol. There are a number of disadvantages for using such water-based heat transfer coupling loops. Electrical power required for pumping water-based heat transfer fluid significantly increases operating costs. Retrofit of aqua-ammonia space conditioning equipment into existing buildings is difficult and expensive requiring replacement of indoor coils and piping typically designed for phase-change refrigerants and, thus, are not sized or designed properly for liquid heat exchange. The temperature glide of a water-based liquid as it exchanges heat between the conditioned space and the outdoor system forces the thermodynamic cycle to operate over an increased temperature lift, thereby reducing system efficiency. In addition, heat exchangers designed for liquid heat transfer are typically larger than those used with phase-change fluids.
Prior-art phase-change assemblies include thermosyphons and heat pipes, neither of which circulates more refrigerant than required to carry the heat load by phase change alone. Overfeed (or overpumping) is the process of circulating more refrigerant in a phase change system such than is required for the thermal load. Overfeed is commonly used in the evaporator loop of industrial and some commercial refrigeration systems with liquid refrigerant returning to a separation tank at the compressor inlet, but is not used in isolated heat transfer assemblies not communicating directly with a compressor. Overpumping has significant advantages in reducing heat transfer surface required, reducing pumping power, and in relaxing or eliminating restrictions on relative elevations of components. Heat pipes and thermosyphons both depend on gravity for circulation and orientation and location of components is important.
The present invention is directed to an improved heat transfer coupling designed to significantly reduce the disadvantages of the hydronic couplings presently used for aqua-ammonia absorption systems, as well as reducing the disadvantages of prior-art phase change coupling. The method and apparatus of the present invention utilize a phase-change refrigerant which is not prohibited nor its use substantially restricted for exchanging heat between the aqua-ammonia system and the indoor or occupied space to be conditioned. The invention includes specific over-pumping of liquid phase-change refrigerant to the heat exchangers of the heat transfer coupling than is required to meet the heat exchange load transfer by phase change alone. A pumped phase-change loop with overpumping is a hybrid of a pumped sensible heat loop and pure phase change loop, and retains advantages of both while avoiding most of their disadvantages. Specific apparatus components and embodiments of the method are described in the following detailed description.